In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
In optical networks, if equipment or a fibre of operating spans is damaged, it is possible to reroute traffic along another path while the damage is repaired. It is also possible to upgrade the equipment in the system without impacting the traffic, again by using alternative optical transmission paths. This has led in some part to optical networks being designed in mesh or ring topologies. In some networks, a secondary optical fibre may be available alongside a primary optical fibre used in normal operating conditions. This secondary optical fibre may be used for operations such as maintenance of the primary optical fibre or restoration if the primary optical fibre is damaged
In optical networks, Raman amplifiers can be used to mitigate attenuation of the optical signal along the path. Raman amplifiers are optical amplifiers based on Raman gain, which results from the effect of stimulated Raman scattering. The optical signal is amplified by providing co-propagating and/or or counter-propagating pump light, usually provided by a pump laser or lasers. The wavelength of the pump light is typically around 100 nanometers shorter than the signal wavelength when using standard c-band channels in a Silica optical fibre as the gain medium.
When optical path switching is used one piece of optical equipment could be used for both the primary and secondary optical fibres, as it is more efficient to be able to switch in these optical fibres to the same equipment, rather than using separate pieces of equipment. FIG. 1 is a schematic illustration of such an amplifier node 100 in an optical network. The amplifier node 100 includes an optical fibre 110 coupled to an amplification device 101 such as an erbium-doped fibre amplifier (EDFA), which may be used in conjunction with the Raman amplifier. A switch 105 is coupled to the fibre 110 outside the amplifier node 100 and configured to switch optical paths between a primary optical fibre 102 and a secondary optical fibre 107. The amplifier node 100 includes a Raman pump unit 120 including one or more light sources such as lasers 103, 104. The lasers 103, 104 supply counter-propagating pump light 111 into the optical fibre 110 to induce Raman gain of an optical signal 108 passing along the primary or secondary optical fibre. The counter-propagating light travels in the opposite direction to the optical signal 108.
During the distributed Raman amplification process, very high optical powers are transmitted directly into the optical fibre 110. As the switch 105 is placed outside the optical amplifier node 100 in FIG. 1, this requires the switch 105 to be able to cope with the high optical power from the lasers 103, 104. In addition, the switch 105 introduces loss and therefore reduces the available pump power in the primary or secondary optical fibre. This arrangement also impacts the depolarisation of the lasers 103, 104, increasing the Degree of Polarisation (DOP) of the lasers and reducing the effectiveness of the Raman amplification process. Furthermore, this arrangement adds risks and safety issues to operators working on or near the optical fibres 102, 107 where very high optical powers are used.
An alternative solution to these problems is to locate the switch within the amplifier node 100. However, in such a case, the switch would still need to support high optical powers, again resulting in high loss. This arrangement would also require a depolarisation technique for the lasers which is difficult to achieve and may need polarisation maintaining (PM) switches.